1. Field of the Invention
The present invention relates to an active matrix type display apparatus which emits light in accordance with an electric current, and in particular, to an active matrix display apparatus using an EL element or the like.
2. Description of the Related Art
In recent years, in a flat display apparatus formed of an electroluminescence (EL) element, it is general that pixels arranged in a matrix form are driven through scanning lines and data lines.
Specifically, pixels are commonly connected to a scanning line on a row basis and to a data line on a column basis. A row scanning circuit selects each of the scanning lines and at the same time a column scanning circuit applies a predetermined display signal to each of the data lines to cause the pixel of the selected row to perform a predetermined display.
An EL display device based on active matrix driving is disclosed in U.S. Pat. No. 6,373,454.
In the display apparatus formed with the EL elements, controlling a current flowing into the EL element enables adjusting the light emitting intensity of each pixel.
FIG. 15 illustrates the schematic diagram of an active matrix type display apparatus. The display apparatus includes a current setting circuit 101, scanning line drive circuit 102 and pixel circuit 103.
FIG. 16 is an example of a pixel circuit including an EL element. Reference characters P1 and P2 denote scanning signal lines and current data “Idata” is input as information signal.
The anode of the EL element is connected to the drain terminal of a TFT (M4) and the cathode thereof is connected to a ground potential CGND.
The pixel circuit further includes p-type TFTs M1, M2 and M4 and an n-type TFT M3. The following briefly describes how the pixel circuit operates.
When the information signal Idata is input, a HI level signal is input into the scanning signal line P1 and a LOW level signal is input into the scanning signal line P2. The transistors M2 and M3 are turned on and the transistor M4 is turned off.
At this point, the transistor M4 is not in a conductive state, which causes a current not to flow into the EL element.
The current data Idata develops a voltage according to the current driving capability of the transistor M1 across a capacitor C1 arranged between the gate terminal of the transistor M1 and the power source potential V1.
When a current needs to be supplied to the EL element, a LOW level signal is input into the scanning signal line P1 and a HI level signal into the scanning signal line P2.
At this point, the transistor M4 is turned on and the transistors M2 and M3 are turned off.
Since the transistor M4 is in a conductive state, a voltage developed across the capacitor C1 supplies the EL element with a current according to the current driving capability of the transistor M1. This causes the EL element to emit light with brightness according to the supplied current.
The above active matrix type EL display apparatus has an unsolved problem that a display on a screen is brightened without increase in consumption power.
As a means of solving the problem, a method is devised of suppressing brightness around the periphery of a screen and increasing it at the central portion thereof. This method exercises a slight influence on display quality.
A related art is disclosed in Japanese Patent Application Laid-Open No. 06-282241, in which the above problem is solved by using a plasma display.
The above art, however, is inapplicable to the active matrix type display apparatus.
Another method is considered in which image data is processed using a lookup table.
In this case, however, a system load is generated when image data is processed and a dynamic range of a data driver needs to be increased.